1. Field of the Invention
This invention relates to deployment of broadband services over the public switched telephone network and more specifically to determining whether the current loop plant of a subscriber can support deployment of broadband services.
2. Background Information
Ubiquitous deployment of broadband services on the loop plant is severely limited by the inherent properties of the copper cable and, in part, because initial deployment of the copper cable was aimed primarily at providing mainly voice services to subscribers. While Fiber To The Curb (FTTC) systems have been built and are commercially available, ubiquitous deployment of FTTC systems necessarily requires upgrading the subscriber loop electronics and the subscriber loop plant at considerable cost and time. Furthermore, Fiber To The Home (FTTH) systems will not only require upgrading the loop plant, but will also require elimination of current loop electronics.
Until the loop electronics and plant are upgraded or replaced, as by installation of optical fiber loops, advanced digital signal processing holds great promise for subscribers who desire broadband services such as high speed internet access, remote Local Area Network (LAN) access and switched digital video today. Over the last several years advanced digital signal processing, creative algorithms and advances in transformers, analog filters and analog to digital (A/D) converters have demonstrated the capability of the copper loop as a transmission channel for significantly higher digital data rates--on the order of millions of bits per second (Mb/s) versus thousand of bits per second (kb/s). These technological advances have brought about Digital Subscriber Line (DSL) technology at high data rates, e.g., High-rate DSL (HDSL), Asymmetric DSL (ADSL) and a variety of other systems that are generically referred to in the art as xDSL. Using ADSL technology, for example, broadband signals are modulated by ADSL modems onto the copper loops at passband frequencies so that Plain Old Telephone Service (POTS) or another baseband service can be carried on the same pair of copper wires. The use of existing copper is extremely cost effective as the installation of new cable and structure along with their associated labor and material costs are avoided. ADSL technology allows telephone companies to rapidly deploy a packet data transport platform that is comparable to the broadband access systems offered by competitors such as cable companies.
Deployment of technologies such as ADSL, however, may be limited by the transmission characteristics of the subscriber loop. The transmission characteristics of the subscriber loop depend on the length of the copper line, its gauge, the presence of bridged taps, the quality of splices, the integrity of the shielding, load coils, impedance mismatches and interference. Specifically, line loss increases with line length and attenuation increases with increasing frequency and decreases as wire diameter increases.
There are particular points along the loop between the subscriber's termination and the originating central office where the loop is particularly susceptible to ingress noise. These points include, for example, the location of a bridged tap, the drop wire from the telephone pole to the home, and the wires within the home. At the aforementioned points ingress noise may be coupled into the loop. The presence of other telephone terminals connected to other pairs in the cable also leads to impulse noise. In addition, bridged taps create more loss, distortion, and echo. All these factors serve to limit the data transfer or information rate at which a subscriber may be connected to a broadband service provider over the subscriber loop and are a major cause of connection problems subscribers currently face in making data connections via the public switched telephone network. In fact, depending on the combination of these factors, a particular loop may not be able to support deployment of broadband technologies such as ADSL at all.
As such, before a particular subscriber can utilize, for example, ADSL technology for his or her broadband services, the broadband service provider (i.e., the service provider who actually offers the broadband service) has to determine or have determined the viability of deploying ADSL to that subscriber. That determination will depend on the length of the copper line, its gauge, the presence of bridged taps and load coils, and interference which, in turn, depend on the physical location of the subscriber's access point and the location of the network termination, e.g., the location of the central office via which the subscriber is provided access to the public switched telephone network.
Currently, when a subscriber requests ADSL service, the service provider has several options to determine the environment the ADSL signal will operate in before they commit to service. The service provider can query the outside plant records to determine the loop configuration. Those outside plant records may be available on an Operations Support System (OSS) and more than likely would have been constructed from the original design records. In many cases, the records available on the OSS are outdated and do not reflect changes that may have occurred in the outside plant as a result of maintenance and service orders. The end result is that the OSS records are usually partially inaccurate and can not be relied upon to provide information required by the carrier to accurately predict a subscriber's loop ability to support ADSL service.
Where the OSS records are known to be grossly inaccurate, it is possible to use the customer's address and the address of the terminating central office and a database of "postal miles" to determine the distance between the customer and central office by road. This information then can be used to estimate the loop length and loop makeup since the postal mileage reflects many of the right-of-ways that are used for installing the outside plant.
Neither of the above approaches provides the information with the degree of accuracy required to confidently predict ADSL performance on a subscriber loop. More importantly, neither of the above approaches are able to estimate the electrical noise environment or distortion that an ADSL or broadband signal may encounter as it traverses the path between the central office and the modem in the subscriber's home.